In an electrophotographic printing machine, the photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostic latent image is recorded on the photoconductive member, the latent image is developed by bringing marking or toner particles into contact therewith. This forms a powder image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the marking particles thereto in image configuration.
Various types of development systems have hereinbefore been employed. These systems utilize two component developer mixes or single component developer materials. Typical two component developer mixes employed are well known in the art, and generally comprise dyed or colored thermoplastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. When the developer mix is brought into contact with the charged photoconductive surface the greater attractive force of the electrostatic latent image recorded thereon causes the toner particles to transfer from the carrier granules and adhere to the electrostatic latent image.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. A light-lens optical system or a raster input scanner (RIS)/raster output scanner (ROS) system may be used to selectively discharge the charged portion of the photoconductive surface to record the repective latent images thereon. Each single color electrostatic latent image is developed with toner particles of a color complimentary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complimentarily colored toner particles. For example, a red light image is developed with cyan toner particles, while a green light image is developed with magenta toner particles and a blue light image with yellow toner particles. Each single color toner powder image is transferred to the copy sheet superimposed over the prior toner powder image. This creates a multi-layered toner powder image on the copy sheet. Thereafter, the multi-layered toner powder image is permanently affixed to the copy sheet creating a color copy. An illustrative electrophotographic printing machine for producing color copies is the Model No. 5775 made by the Xerox Coporation.
It is evident that in printing machines of this type, toner particles are depleted from the developer mixture. As the concentration of toner particles decreases, the density of the resultant copy degrades. In order to maintain the copies being reproduced at a specified minimum density, it is necessary to regulate the concentration of toner particles in the developer mixture. This is achieved by a closed loop servo system which regulates developability. Developability, as it pertains to an electrophotographic printing machine is the ability of the developer mixture to develop the latent image with at least a minimum specified density. It has long been recognized that a closed loop system, which regulates developability by measuring the density of the powder image developed on the photoconductive surface, optimizes cost and performance. This is possible because of the relative stability of the other steps in the imaging process such as transfer and fusing. Also, by modulating one parameter, such as toner particle concentration, compensation for factors contributing to low copy quality, such as photoreceptor dark decay fluctuation and developer aging, can be partially accomplished. The use of densitometers for measuring the optical density of black toner particles is well known. However, densitometers used for black toner particles are generally unsuitable for use with colored toner particles because they are generally sensitive to the large component of diffusely reflected flux in the infrared light from colored toner particles, which gives false density measurements.
In measuring the density of toner on a photoreceptor surface, even in the case of black toner, a crucial consideration is the ability to measure the specularly reflected light from the surface of the partially covered photoreceptor, while excluding light reflected as a result of the reflectivity of the toner itself. Generally, no matter what the color of a particular type of toner, the more densely the toner is applied to the photoreceptor, the "darker" (more absorptive, less reflective) the toner will appear on the photoreceptor up to some maximum saturation value. When the purpose of a densitometer is to measure the density of toner on a surface, the most important measured value is the light directly reflected from the surface, which is the difference between the light transmitted to the surface and the light blocked by the toner. However, because toner is not only absorptive but partially reflects light itself, a quantity of "scattered" or "diffuse" light will be returned to the detector from the toner as well. The light scattered by the toner itself will interfere with a reading of that blocked by the toner. Thus, it is desired that a system be devised to measure only the fraction of light blocked by toner on the photoreceptor surface, without interference from the scattered light reflecting from the toner itself, which can be considered a type of noise.
Various approaches have been used to measure directly-reflected light from toner on a surface, in order to infer therefrom the absorption of light and therefore toner density. The following disclosures appear to be relevant:
U.S. Pat. No. 4,553,033. Patentee: Hubble, III et al. Issued: Nov. 12, 1985.
U.S. Pat. No. 4,750,838. Patentee: DeWolf et al. Issued: Jun. 14, 1988.
U.S. Pat. No. 4,796,065. Patentee: Kanbayashi. Issued: Jan. 3, 1989.
U.S. Pat. No. 4,799,082. Patentee: Suzuki. Issued: Jan. 17, 1989.
U.S. Pat. No. 4,801,980. Patentee: Arai et al. Issued: Jan. 31, 1989.
U.S. Pat. No. 4,989,985. Patentee: Hubble, III et al. Issued: Feb. 5, 1991.
U.S. Pat. No. 5,083,161. Patentee: Borton et al. Issued: Jan. 21, 1992.
The relevant portions of the foregoing patents may be briefly summarized as follows:
U.S. Pat. No. 4,553,033 discloses an infrared reflectance densitometer including a light emitting diode, a collimating lens through which light is projected to a photosensitive surface, a collector lens and a field lens through which reflected light is focused onto a signal photodiode, and a control photodiode onto which a portion of reflected light is directed to control light output. The amount of light received on the signal photodiode is a measurement of the reflectance from the surface of the photoreceptor which, in turn, is proportional to the density of the toner particles thereon.
U.S. Pat. No. 4,750,838 describes an optoelectric circuit for measuring differences in optical densities of an image carrier. An LED illuminates a test area. The light reflected from the surface is sensed by a phototransistor. The linear output of the LED is proportional to the image density. The circuit has a voltage follower, output transistor, amplifier and differential amplifier for controlling the image density measurements. The circuit has a range of density sensitivities between 0.0 and 1.5 mg/cm.sup.2.
U.S. Pat. No. 4,796,065 discloses an apparatus for detecting image density in an image forming machine by sensing either regular reflection or scattered reflection. A circuit having light emitting elements (LEDs or phototransistors), a pair of sensors, and a comparator is used for determining image density.
U.S. Pat. No. 4,799,082 describes an electrostatic reproducing apparatus having a light source and detector for detecting color toner density. A sensor is driven by a circuit which contains a power source, a safety resistor, operational amplifier, comparator and voltage dividing resistors for producing a signal reprsentative of the light reflected from the image.
U.S. Pat. No. 4,801,980 discloses a toner density control apparatus which compares an image density of a reference image with a predetermined level to control density. Voltage to a light emitting element is controlled by the circuit which includes a sensor correction portion.
U.S. Pat. No. 4,989,985 describes an infrared densitometer which measures the reduction in the specular component of reflectivity as toner particles are progressively deposited on a moving photoconductive belt. Collimated light rays are projected onto the toner particles. The light rays reflected from at least the toner particles are collected and directed onto a photodiode array. The photodiode array generates electrical signals proportional to the total flux and the diffuse component of the total flux of the reflected light rays. Circuitry compares the electrical signals and determines the difference therebetween to generate an electrical signal proportional to the specular component of the total flux of the reflected light rays.
U.S. Pat. No. 5,083,161 describes an infrared densitometer which measures the reflectivity of a selected region on a surface by reflecting light rays from a single source off the selected region onto an array of photodiodes.